Because of the physical characteristics of aluminum pistons it has been the practice to manufacture pistons with built-in cast iron inserts. These annular inserts are provided with one or more peripheral grooves of precise cross sectional configuration for receiving a corresponding number of cast iron piston rings. During the manufacturing process molten aluminum material is poured around the inserts, and in order to obtain the desired degree of bond therebetween as the aluminum and insert materials cool down through a temperature range of about 770.degree. C. (1380.degree. F.) the insert material has been heretofore limited to austenitic gray cast iron. This choice is directly due to the fact that an austenitic microstructure exhibits a coefficient of thermal expansion rate more directly approaching that of the aluminum alloy material of the piston and, hence, a better bond is theoretically possible.
However, in order to extend the service life of engines the piston rings have been made harder. This has desirably resulted in a reduced rate of piston ring wear, but has undesirably resulted in an increased rate of wear and/or deformation of the insert grooves.
Thus, what is desired is an economical method of improving the hardness level of such an insert for improved service life, while still providing a highly effective bond between the material of the piston and the material of the insert during both the manufacturing stages and the dynamic operating conditions experienced in an internal combustion engine.
The present invention is directed to overcoming one or more of the problems as set forth above.